(1) Field of the Invention
The present invention relates to a method and system for driving a high-pressure mercury vapor discharge lamp having a pair of discharge electrodes in an arc tube in which mercury and a rare gas are enclosed. The invention further relates to an image projector employing the lamp.
(2) Description of the Prior Art
High-pressure mercury vapor discharge lamps are known to have a high luminance and therefore utilized for a light source of such optical devices as a liquid crystal projector and the like, with the lamp being positioned so that the light emitting region (arc) is located at the focal point of a reflector mirror such as a parabolic mirror. When such a high-pressure mercury vapor discharge lamp is used as a component in such optical devices, it is necessary that loss of the light emitted from the lamp be minimized so that a high illuminance on the screen is achieved. For this reason, it is preferable that a lamp used for such a light source of liquid crystal projectors have a light emitting region as close as possible to a point source. More specifically, so-called short arc lamps, which have a short light emitting region, are preferable, and in addition, high-pressure mercury vapor discharge lamps are more preferable than metal halide lamps.
In metal halide lamps, the metals enclosed therein as metal halides have a low average excitation energy, and therefore the lamps operate at a relatively low arc temperature. As a result, the light emitting region is not restricted in a region between the electrodes but is spread over a wide region from the center of the arc tube towards the arc tube wall, and accordingly, a point source light is difficult to be achieved by using a metal halide lamp. By contrast, in high-pressure mercury vapor discharge lamps, mercury, which has a high average excitation energy, is enclosed as a fill material thereof. Therefore, in high-pressure mercury vapor discharge lamps, the light emitting region is restricted in a small region, and thus, a light emitting region close to a point source is readily achieved. An example of such a short arc high-pressure mercury vapor discharge lamp is disclosed in Japanese Unexamined Patent Publication No. 2-148561. In this lamp, a mercury vapor pressure during lamp operation is made to be 200 atm. or higher, and continuous radiation components in the visible light range are thus generated by mercury molecule emission. The lamp has a lamp power of 50 W, and exhibits a high luminance and improved color rendering property, and is therefore suitable for the light source of liquid crystal projectors.
As for methods of driving such high-pressure mercury vapor discharge lamps used for a liquid crystal projector and the like, special operating conditions such as an instant restarting and an instant starting of light rays necessary for motor vehicle headlights are not required, and therefore, the driving methods are seldom described in publications. Generally, for driving a high-pressure mercury vapor discharge lamp, alternating voltage having a frequency of several tens to several hundreds hertz is used, since within the frequency range, circuit designing is rendered relatively easy in view of the response speed of large power semiconductor devices.
With a recent trend toward a larger screen size and a higher resolution of liquid crystal projectors, a lamp having a large lamp power that can achieve a higher illuminance on the projection screen has been increasingly demanded. In order to achieve such a lamp, it may be possible to increase a lamp power by raising a mercury vapor pressure during lamp operation and thereby increasing a lamp voltage. However, when rising a mercury vapor pressure is difficult because of the constraint imposed by a wall tube strength against pressure, a lamp current must be increased to increase the lamp power.
In the case in which a high-pressure mercury vapor discharge lamp is operated with a large lamp current, however, there arises a problem that flicker is caused when the lamp is operated by the foregoing conventional driving method with alternating voltage having a frequency of several tens to several hundreds hertz. Flicker is a phenomenon in which an illumination of a projected image is varied from moment to moment. Such flicker induces poor quality in the projected image in liquid crystal projectors and the like employing a high-pressure mercury vapor discharge lamp.
In view of the foregoing and other drawbacks in prior art, it is an object of the present invention to provide a method and a system for driving a high-pressure mercury vapor discharge lamp for use in a liquid crystal projector and the like, which method and system are free from flicker in the projected image even when the lamp is operated with a large lamp current.
It is another object of the invention to provide an image projector using such a driving system.
In order to achieve the above and other objects, the inventors have studied the cause of such flicker, and as a consequence discovered that wandering of a cathode luminescent spot generated in the vicinity of a tip of each electrode is the cause of the flicker. Referring now to FIG. 1 showing a luminance distribution of the light emitting region between the electrodes 11 and 12, it is noted that a cathode luminescent spot refers to a spot at which the highest luminance is observed (the reference numerals 11a and 12a in FIG. 1) in the vicinity of a point at which electrons are emitted when each of the electrodes 11 and 12 is turned to be a cathode. Now, discussed below is a process in which the wandering of a cathode luminescent spot occurs.
For example, as shown in FIG. 2A, when a negative voltage is applied to the electrode 11 and a positive voltage to the electrode 12, a minute region 11a adjacent to the tip of the electrode 11 is heated to a high temperature and thus turned to be a cathode luminescent spot at which arc is generated by thermionic emission effect. By emission of electrons, such a state of the minute region 11a being a high temperature is further maintained. Thereafter, as shown in FIG. 2B, when the polarity of the applied voltage is reversed, the electrons emitted from the electrode 12 enters a wide region in the tip of the electrode 11. By the entry of the electrons, the energy from the electrons is transferred to the electrode 11, and the wide region of the tip of the electrode 11 is heated as well as the minute region 11a. 
Here, in the case of a lamp current being relatively small, although the electrode 11 is heated, the state of the minute region 11a having a higher temperature than the rest of the region is still maintained. As a consequence, when the polarity of the applied voltage is again reversed, the minute region 11a is again turned to be a cathode luminescent spot. Therefore, once a cathode luminescent spot forms at the minute region 11a, the spot does not easily move to another region but stays at a relatively stable position. By contrast, in the case of a lamp current being large, more specifically as shown in FIG. 2C, in the case where a lamp current is so large that the electrode 11 is heated in a wide region 11b in the electrode tip and, viewed on a so-called microscopic level, the electrode 11 is fused and deformed by the heat from moment to moment, the minute region 11a is not necessarily in the state of a higher temperature than that of the rest of the region. In other words, any spot in the wide region adjacent to the tip of the electrode 11 can become a temperature that can result in a cathode luminescent spot. Consequently, it can occur that, when the polarity of the applied voltage is again reversed, another minute region 11c, not the minute region 11a, is turned to be a cathode luminescent spot. That is to say, influenced by convection in the arc tube or surface roughness of the tip of the electrode 11 caused by the heat deformation, a position of a cathode luminescent spot tends to frequently wander in the vicinity of the tip of the electrode 11. Such wandering of the cathode luminescent spot occurs either periodically or non-periodically. In addition, such wandering of the cathode luminescent spot can occur in the electrode 12 as well.
Such wandering of the cathode luminescent spot induces a variation of a luminance distribution in the light emitting region between the electrodes 11 and 12. Note here that, due to the fact that the light emitting region is not a point-source but has a finite volume, a liquid crystal projector and the like is generally configured such that light rays from the various spots in the light emitting region with a certain size are reached and superposed on the projected screen. For this reason, when a luminance distribution of the light emitting region is varied, an illuminance on the projected image is accordingly varied. Such an illuminance variation particularly raises a serious problem in the case of using a short arc lamp. Generally, in a so-called long arc lamp, the shape and position of the arc is regulated by the arc tube wall, and therefore wandering of a cathode luminescent spot is not easily caused. Even if the wandering is caused, undesirable effects on the illuminance variation is not considerable since the area of wandering is small relative to the arc length.
The present inventors have also carried out a study about in what cases a viewer recognizes the illuminance variation as described above as flicker. As a result of the study, it has been found that when a variation of illuminance exceeds xc2x15% of the immediately preceding illuminance and occurs approximately 60 times/second or less, the variation of illuminance is recognized as flicker, which causes a poor image quality.
In view of the foregoing and other problems in the prior art, the present inventors sought a method for suppressing wandering of a cathode luminescent spot in order to prevent flicker in the projected image, and thus have accomplished the present invention.
The foregoing and other objects are accomplished in accordance with the present invention, by providing a method for driving a high-pressure mercury vapor discharge lamp comprising in an arc tube a pair of discharge electrodes opposed to each other, the lamp wherein at least mercury and a rare gas is enclosed in the arc tube, comprising a step of:
applying an alternating voltage between the discharge electrodes, the alternating voltage having a frequency, for example in the range of 20 kHz to 42 kHz, such that wandering of a cathode luminescent spot generated in the vicinity of a tip of each of the discharge electrodes is suppressed.
The above-described method for driving a high-pressure mercury vapor discharge lamp may be such that:
the high-pressure mercury vapor discharge lamp has an arc length and a rated power such that
P/dxe2x89xa780 (W/mm), 
where d is the arc length (mm) and P is the rated power (W).
By employing a driving method as described above in which the foregoing alternating voltage is applied to a lamp, wandering of a cathode luminescent spot is suppressed. This makes it possible to prevent flicker in the projected image resulting from the wandering of a cathode luminescent spot in a high-pressure mercury vapor discharge lamp used for a liquid crystal projector and the like.
The present invention also provides a driving system for a high-pressure mercury vapor discharge lamp comprising in an arc tube a pair of electrodes opposed to each other, the lamp wherein at least mercury and a rare gas are enclosed in the arc tube, the driving system operating the lamp by applying an alternating voltage between the discharge electrodes, wherein:
a frequency of the alternating voltage is such that wandering of a cathode luminescent spot generated in the vicinity of a tip of each of the discharge electrodes is suppressed, for example in the range of 20 kHz to 42 kHz, and more specifically, the frequency of the alternating voltage is such that, when an image of an arc adjacent to a tip of each of the discharge electrodes is projected onto a predetermined projection plane, a variation of an illuminance on the projection plane is xc2x15% or smaller.
The above-described driving system for a high-pressure mercury vapor discharge lamp may be such that:
the high-pressure mercury vapor discharge lamp has an arc length and a rated power such that
P/dxe2x89xa780 (W/mm), 
where d is the arc length (mm) and P is the rated power (W), or the high-pressure mercury vapor discharge lamp has an arc length of 3 mm or less, or
a temperature of a tip of each of the discharge electrodes is 3000 K or higher in the high-pressure mercury vapor discharge lamp being. operated.
By employing the above-described driving system in which the foregoing alternating voltage is applied to a lamp, wandering of a cathode luminescent spot is suppressed. This makes it possible to prevent flicker in the projected image resulting from the wandering of a cathode luminescent spot in a high-pressure mercury vapor discharge lamp used for a liquid crystal projector and the like.
The above-described driving system for a high-pressure mercury vapor discharge lamp may further comprise:
means for adjusting the frequency of the alternating voltage, and
means for detecting a luminance in the vicinity of a tip of the discharge electrodes,
the driving system wherein:
the means for adjusting the frequency of the alternating voltage controls the frequency of the alternating voltage to be a frequency such that wandering of a cathode luminescent spot generated in the vicinity of a tip of each of the discharge electrodes is suppressed, in response to a result detected by the means for detecting.
By employing the above-described driving system, even in the case where the lamp has varied characteristics or induces a variation due to aging, it is ensured that wandering of a cathode luminescent spot is suppressed and thereby flicker in the projected image is prevented.
The present invention further provides an image projector comprising:
a high-pressure mercury vapor discharge lamp comprising in an arc tube a pair of discharge electrodes opposed to each other, the lamp wherein at least mercury and a rare gas is enclosed in the arc tube,
a driving system for operating the high-pressure mercury vapor discharge lamp by applying an alternating voltage between the discharge electrodes, and
a projection optical system using a light emitted from the high-pressure mercury vapor discharge lamp as a source light to project an image onto a projection screen,
the image projector wherein:
a frequency of the alternating voltage applied to the high-pressure mercury vapor discharge lamp by the driving system is such that wandering of a cathode luminescent spot generated in the vicinity of a tip of each of the discharge electrodes is suppressed.
By the image projector of the above-described configuration, it is made possible to suppress wandering of a cathode luminescent spot and thereby to prevent flicker in the projected image, and thus high quality image can be achieved.